I am going on the new IB J class to MAD in a bout a month, looks lovely. The retrofitted planes have their own designation as A340-500 or A345 (I think) I understand this is a plane that was never built and is only used to identify which IB A340s have the retrofit. So if you can find a website that tells you that information, then you can make sure you get the retrofitted plane.

You can also consider AV LHR BOG LIM. Their A330s are also being retrofitted/delivered new with a horizontal flat-bed herringbone layout. No way of guaranteeing you will get it but the first A330 delivered with this configuration has been doing a lot of London rotations